I fiddled with the NBT tags of the two pigs. Texelelf's NBT Editor allows you to edit NBT tags without leaving MCEdit. I'd say that the interface is slightly worse than NBT Explorer, but it makes up for that in spades by being directly linked to MCEdit. Halves the time it takes to get things done.

Basically, one of the new sets of tags, Leash, uses XYZ co-ordinates when tied to a fencepost, but UUIDs when being pulled by a player. Every object in the game has a UUID (I think), including players, and they are all unique. So, what I did was close the game whilst dragging the lower pig, head over to the Leash data, and switch out my UUID for that of the upper pig. I then made sure to also leash the upper pig to the lower pig, and, hey presto, they're linked!

As far as I know, the NBT data that defines what a spawner produces only works on a single entity and can't be used to spawn multiple connected entities eg. spider jockeys or in this case, two pigs tethered together.

The NBT data for leashes involves Universally Unique Identifiers, so while you probably can't safely spawn in the mob you want to leash things to, you can still spawn in the leashed mobs (even pre-leashed to a given fencepost or something).

I'm not the creator, but yes this would work in multiplayer, and yes you could make it a hanging roller coaster of some kind. Though it may glitch out in multiplayer, I don't think so as single player is just multiplayer ran locally.